[PATCH V12 03/15] iothread: track users with holder name

Introduce iothread_ref_and_get_aio_context() with a holder argument
and its counterpart iothread_unref_and_put_aio_context().

Previously, users of an IOThread AioContext did not explicitly record
their identity, making it difficult to debug which devices or
subsystems were pinning an IOThread.

Registering a holder takes an IOThread object reference so that the
IOThread and its AioContext stay alive until the matching put
operation. Document the ownership and BQL requirements.

Signed-off-by: Zhang Chen <[email protected]>
---
 include/system/iothread.h | 22 ++++++++++++++++++++++
 iothread.c                | 24 +++++++++++++++++++++++-
 2 files changed, 45 insertions(+), 1 deletion(-)

diff --git a/include/system/iothread.h b/include/system/iothread.h
index b8aeb32b0e..f6c5f95dd9 100644
--- a/include/system/iothread.h
+++ b/include/system/iothread.h
@@ -67,6 +67,28 @@ DECLARE_INSTANCE_CHECKER(IOThread, IOTHREAD,
 char *iothread_get_id(IOThread *iothread);
 IOThread *iothread_by_id(const char *id);
 AioContext *iothread_get_aio_context(IOThread *iothread);
+
+/*
+ * Register @holder and return @iothread's AioContext.  The holder is copied,
+ * and a reference is taken on @iothread so that both the IOThread and its
+ * AioContext remain alive.
+ *
+ * The caller must eventually call iothread_unref_and_put_aio_context() with an
+ * equivalent holder.  This function is not thread-safe and must be called
+ * under the Big QEMU Lock (BQL).
+ */
+AioContext *iothread_ref_and_get_aio_context(IOThread *iothread,
+                                             const IOThreadHolder *holder);
+
+/*
+ * Unregister @holder and release the corresponding reference on @iothread.
+ * Calling this function without a matching
+ * iothread_ref_and_get_aio_context() call is a programming error.
+ *
+ * This function is not thread-safe and must be called under the BQL.
+ */
+void iothread_unref_and_put_aio_context(IOThread *iothread,
+                                        const IOThreadHolder *holder);
 GMainContext *iothread_get_g_main_context(IOThread *iothread);
 
 /*
diff --git a/iothread.c b/iothread.c
index 2a4c92e08b..64be852d1c 100644
--- a/iothread.c
+++ b/iothread.c
@@ -33,6 +33,11 @@ void iothread_ref(IOThread *iothread, const IOThreadHolder *holder)
     assert(holder);
 
     QAPI_LIST_PREPEND(iothread->holders, QAPI_CLONE(IOThreadHolder, holder));
+    /*
+     * This guarantees that the IOThread and its AioContext remain alive
+     * as long as there is a holder.
+     */
+    object_ref(OBJECT(iothread));
 }
 
 static int iothread_holder_compare(const IOThreadHolder *holder_a,
@@ -78,6 +83,7 @@ void iothread_unref(IOThread *iothread, const IOThreadHolder *holder)
             *prev = curr->next;
             curr->next = NULL;
             qapi_free_IOThreadHolderList(curr);
+            object_unref(OBJECT(iothread));
             return;
         }
         prev = &curr->next;
@@ -199,7 +205,7 @@ static void iothread_init_gcontext(IOThread *iothread, const char *thread_name)
     g_autofree char *name = g_strdup_printf("%s aio-context", thread_name);
 
     iothread->worker_context = g_main_context_new();
-    source = aio_get_g_source(iothread_get_aio_context(iothread));
+    source = aio_get_g_source(iothread->ctx);
     g_source_set_name(source, name);
     g_source_attach(source, iothread->worker_context);
     g_source_unref(source);
@@ -421,6 +427,22 @@ AioContext *iothread_get_aio_context(IOThread *iothread)
     return iothread->ctx;
 }
 
+AioContext *iothread_ref_and_get_aio_context(IOThread *iothread,
+                                             const IOThreadHolder *holder)
+{
+    /* Add IOThreadHolder to the list */
+    iothread_ref(iothread, holder);
+
+    return iothread->ctx;
+}
+
+void iothread_unref_and_put_aio_context(IOThread *iothread,
+                                        const IOThreadHolder *holder)
+{
+    /* Delete IOThreadHolder from the list */
+    iothread_unref(iothread, holder);
+}
+
 static int query_one_iothread(Object *object, void *opaque)
 {
     IOThreadInfoList ***tail = opaque;
-- 
2.54.0